In semiconductor wafer processing operations, as in many other mass processing operations, an undetected process error can possibly ruin an entire process lot of potential product. Reliable process controls appear necessary to avoid turning successful operations into disaster. However, process controls also need to be cost effective in that they may not unduly raise the cost of the ultimate product.
The present invention has been found to be of particular use in photolithographic operations which, according to current manufacturing techniques, precede most of the selective treatments of semiconductor wafers by which devices are formed in such wafers. In such photolithographic operations, a layer of photoresist is applied over the entire wafer. The applied layer of photoresist is first dried or baked, then typically selectively exposed and developed to open selected windows in the layer through which underlying areas of the wafer will be treated in a subsequent operation. During such treating operation, the wafer areas beneath the remaining photoresist layer remain protected.
Photoresist layers at times include defects in the form of pinholes extending through the applied layer. Pinholes tend to occur sporadically. When a pinhole exists in an area of the resist layer which is to be developed away during a subsequent operation, there is no adverse effect on the final product. If, however, the pinhole resides in an area of the resist layer which needs to remain intact to protect the underlying area of the wafer during a subsequent operation on the wafer, then the respective ultimate device is likely to be defective.
A cause for the occurrence of pinholes in the photoresist layers is not always determinable. Otherwise their occurrence could be avoided. However, various causes by which pinholes may occur are known such as the presence of small areas of contamination on the base layer of a semiconductor wafer to which the photoresist is applied. Areas of contamination may cause the wafer to be repellent to the photoresist such that a pinhole occurs in the photoresist above such contamination. Pinholes may also occur through particulate contaminations either on the surfaces of the wafers or in the photoresist.
Under normal process conditions pinholes are not expected to occur in significant numbers to produce serious yield affecting densities. However, since the sudden occurrence of pinholes may cause a significant loss of product, a statistical surveillance test has been adopted. According to the test, special test wafers are included in each lot of wafers being coated with a layer of photoresist. The test wafers typically have a dielectric sublayer of oxide onto which the photoresist is applied. After a typical drying operation, the test wafers are subjected to an oxide etch operation after which the photoresist is stripped and the oxide layer is inspected for pinholes.
During the oxide etch operation, the photoresist layer protects the oxide layer except where a pinhole exists in the photoresist layer and permits a pinhole-sized aperture to be etched through the oxide layer to the wafer itself. Pinholes are virtually impossible to detect even when the wafers are inspected through a microscope. The pinholes are small enough to typically blend in with other surface features and are consequently overlooked in a microscope scanning operation.
The operation of a commercially available pinhole density test apparatus helps to identify otherwise hard to detect pinholes. A wafer to be scanned through image enlarging optics, such as those found on a microscope or on an enlarging video camera, is mounted horizontally in a special wafer holder. The holder features a peripheral ridge about the wafer to permit the upward facing front surface of the wafer to be covered with an electrolytic fluid. An electrical contact to the backside of the wafer couples the wafer to an electrolytic circuit which is completed through any pinholes which might extend through the oxide covering the front surface of the wafer. In a typical embodiment of the test apparatus, an annular electrode touches the surface of the electrolytic fluid in a fringe area about the field of view of the scanning optics and remains stationary with respect to the optics. Consequently, any electrolytic action on the surface of the wafer due to pinholes through the oxide layer is concentrated in the field of view of the scanning optics.
The field of view is typically moved with respect to the wafer in a raster pattern which ultimately covers the entire wafer but without overlap. An electrolytic action has been found to cause gas bubbles to rise from pinholes. Such bubbles are much more visible than the pinholes themselves. In addition, typically a string of such bubbles rises through the fluid, like a streamer which points at its origin to the pinhole.
Nevertheless, scanning the test wafers in the described manner is time consuming and costly. Typically an entire production lot of wafers which had been subjected to the same resist coating operation is held up between the photoresist application and a subsequent selective treatment. On the basis of the pinhole count on the test wafer, a decision is made whether to strip and re-apply the photoresist on the production lot or whether to continue the process with the already applied resist.
It is now realized that economies can be obtained by reducing the delay in process time which is needed to determine whether the photoresist layer is of acceptable quality. Because typical semiconductor device processes are series of selective process steps, which repeatedly include photoresist operations, the total delay in the throughput of the device manufacture amounts to a considerable time interval.